Low NOx regenerative burner

ABSTRACT

A regenerative burner having heat storage units with combustion effluent/combustion air ducts therethrough, fuel intake means and a burner body, wherein the burner is designed to suppress NOx formation and to control flame shape and characteristic in the regenerative system during combustion. The regenerative burner may include a burner baffle, or may include a plurality of gas jets entrained in generally converging fashion for control of the flame characteristics and shape dispositive of NOx formation. The burner may provide for staged combustion, either by means of sequential fuel injection or sequential provision of combustion air, or the burner may depress NOx formation by vitiation of combustion air with products of combustion. The present regenerative burners suppress NOx formation yet preserve the remaining characteristic features of regenerative systems.

FIELD OF THE INVENTION

The invention relates to regenerative type burners, for heating afurnace, with minimized NOx formation in the ultimate combustioneffluents.

INTRODUCTION

Regenerative-type burners for furnaces are well-known in the art invaried forms and designs, but they share the common feature whereby heatstorage units are provided to trap and store heat from combustioneffluents, with subsequent transfer of the heat to preheat combustionair. The earliest regenerative-type furnaces were symmetricalarrangements having both burner(s) and heat storage units (often solidstructural arrays of "checker chamber" bricks) in place on each of twosides of the furnace. Firing of such a regenerative furnace began withthe burner(s) on one side, with concomitant storage of the heat presentin the combustion effluents by the heat storage units on the secondside. After optimal heating of the heat storage units, or the "checkerchamber," the air flow in the furnace was reversed to draw combustionair in through the checker chamber, thus preheating the combustion air.Ducts in the checker chamber thus alternately conveyed combustionproducts and combustion air, and the burners functioned alternately asburners and as flues.

Modern regenerative systems do not involve complete symmetrical furnacesbut instead include specialized regenerative burners employed,typically, in pairs. Each of the paired regenerative burners is equippedwith heat storage units, ordinarily in the form of compact regenerativebeds, through which combustion air passes en route to the burner.Because the burners are employed in pairs, one burner is fired at a timewhile the other functions as a flue and heat storage bed. Then, every20-120 seconds or so, flow in the furnace is reversed and the burners"exchange" functions the first-fired burner becomes the flue/heatstorage bed as the second burner fires A system exemplary of one pairedburner arrangement is found in U.S. Pat. No. 4,522,588.

A persistent problem with regenerative systems involves the extremelyhigh NOx concentrations inevitably present in the combustion effluents,produced as a result of the extremely high air preheats and flametemperatures, as well as through fuel bound nitrogen As a result,regenerative systems which historically enjoyed industry-wide acceptancenow cannot meet the ecology standards in an ever-increasing number ofcountries and/or process conditions Additionally, the burners used withprior art "regenerator pairs" are of a fixed design and are notadaptable to control flame shape or characteristic. There is a need forlow NOx burner concepts which can be broadly adapted to the specificapplications by altering the flame shape or characteristic to meet theexact requirement. A need therefore persists for regenerative burnersystems which provide the heat-regenerative function of prior artsystems yet provide for significant NOx reduction and applicationadaptability as well.

SUMMARY OF THE INVENTION

In order to meet this need, the present invention is a regenerativeburner having fuel delivery means, a burner body and a regenerative bedconnected to a combustion air/exhaust passage, wherein the burner isdesigned to suppress NOx formation in the regenerative system duringcombustion and can readily be arranged to control the flame shape andcharacteristic. NOx can be substantially reduced by incorporating in theburner design a suitable baffle or, more broadly, means to inducerecirculation of combustion gases back into the primary combustion zone,by staging the fuel through sequential fuel injection, by staging thecombustion air through sequential introduction of that air or throughthe use of vitiated combustion air where products of combustion aremixed with the combustion air on a controlled basis to reduce the oxygencontent of the air, either by mixing flue gas into the combustion air atthe fan inlet or within the burner structure itself.

The regenerative burner may, therefore, include a burner baffle with airjets or may include a plurality of gas jets, in such a manner that thejets induce recirculation of combustion gases back into the primarycombustion zone to suppress NOx and also act to control the fuel/airmixing rates which establish flame shape and characteristic. The burnermay provide staged combustion, either gas or air, or may suppress NOxthrough vitiating the air within the burner or through the use ofvitiated air from the combustion fan. Certain embodiments of the presentregenerative burner may include combustion gas recirculation back intothe primary combustion zone, the use of vitiated air and either fuel orcombustion air staging. The present regenerative burners reduce NOx,control the flame shape and characteristic, yet preserve the highthermal efficiency characteristics of the regenerative systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a furnace and two low NOx regenerativeburners, also showing the method of vitiating the air at the fan inlet;

FIG. 2 is a sectional view of a first embodiment of a low NOxregenerative burner having a burner baffle;

FIG. 2a is a section taken along lines 2a--2a of FIG. 2;

FIG. 3 is a sectional view of a second embodiment of a regenerativeburner having fuel injection nozzles for combustion air and combustiongas entrainment;

FIGS. 4 and are sectional views of two regenerative burners (third andfourth embodiments) adapted for staged combustion;

FIG. 6 is a sectional view of a fifth embodiment of a regenerativeburner in which combustion air is vitiated with products of combustion;

FIGS. 7 and 9 are sectional views of a sixth and seventh embodiment of aregenerative burner having pairs of converging nozzles;

FIG. 8 is a sectional view taken along lines VIII--VIII of FIG. 6; and

FIG. 10 is a sectional view of an eighth embodiment of a regenerativeburner in which first and second concentric streams (fuel/fuel,fuel/air, fuel/products of combustion) are introduced into thecombustion site.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments of the invention are best considered first in context,with initial reference being to the schematic diagram of FIG. 1. In FIG.1, a regenerative system 10 is illustrated in which a furnace chamber 12is equipped with a pair of regenerative burners 14 as shown. Eachregenerative burner 14 is connected to an immediately adjacentregenerative bed 18, through which combustion air/exhaust pass betweenthe burner 14 and the combustion air/exhaust passage 16. Combustion airis provided to one burner 14 at a time by action of the combustionblower 20, so that when the left burner 14 (shown in FIG. 1) is firing,the left combustion air valve 22 is open and the left exhaust valve 24is closed, with the right combustion air valve 22 and right exhaustvalve 24 closed and open, respectively, to enable the right burner 14 tofunction as a flue for the furnace chamber 12. As a result, when theleft burner 14 fires, the regenerative bed 18 of the right burner 14collects heat from the combustion effluents. When flow in the furnace isreversed, the right regenerative bed 18 preheats combustion air for theleft regenerative burner 14. Connection 26 shown with the control valve27 provides the means for vitiating the air with products of combustionat the fan inlet 21.

Referring now to FIG. 2, the regenerative burner as illustrated includesa burner 30, from which a fuel nozzle 32 (embedded in refractory) fuelscombustion in the port 34. Alternatively, the fuel nozzle may beinsulated or air cooled by other means known in the art. A pair ofburners (only one shown) provide combustion for an adjacent furnacechamber as shown in FIG. 1 and as known in the art. Combustion air isprovided to the burner 30 via air uptake 36, which passes through theregenerative bed 38 as shown. The fuel nozzle 32 is supplied by a fuelline 40. Disposed adjacent the fuel nozzle 32 is a burner baffle 42,which channels the flow of combustion air into the port 34.

The structure of the burner baffle 42 is more readily seen in FIG. 2a,which is a section taken along line 2a--2a of FIG. 2. The burner baffle42 is a generally cylindrically shaped structure having four burnerbaffle apertures 44 therein, spaced as shown. (In FIG. 2, the burnerbaffle apertures 44 are shown above and below the nozzle end of the fuelline 40; the exit of each aperture 44 is coplanar with the tip of thefuel nozzle 32.) As shown in FIG. 2a, air passage is prevented exceptthrough the burner baffle apertures 44. The jet effect of the combustionair introduced through the four apertures immediately upstream of thefuel nozzle 32 creates a low pressure region at the baffle face whichinduces recirculation of combustion gases back into the primarycombustion zone, thus lowering the flame temperature and substantiallyreducing NOx levels in the combustion effluents of the burner. The useof four apertures, spaced as shown, provides adequate recirculationregions for the combustion gases between the holes. The same combustionair jet action induces fuel flow to the combustion air, providing therequired mixing of fuel and air which strongly influences flame shapeand characteristic. The angularity and direction of the combustion airholes can be adjusted to control flame shape and characteristic.Although the four-hole burner baffle 42 is preferred from the standpointof minimized NOx, the number and arrangement of the combustion air holescan be varied as required and incorporated into the regenerative burner30 of FIG. 2.

Dimensions for the burner baffles 42 will vary, but an exemplary baffleis 61/2 inches in outer diameter, with proportionally sized apertures asshown. Baffle diameters commonly range between 5 and 30 inches. Althoughthe four-hole burner baffle 42 is preferred, as the baffle whichmaximizes NOx reduction, 6-, 8-, 9- and 12-hole baffles also reduce NOxand may therefore also be incorporated into the regenerative burner 30of FIG. 2.

Referring once again to FIG. 2, and as applies to all other embodimentsof the invention, the burner 30 is constructed of a fabricated metalouter casing (not shown) and is fully insulated with a suitableinsulating material. The fuel lines and nozzles are fabricated ofstandard materials including metals, and such metal structures areeither adequately insulated or air cooled when fully exposed (see FIGS.2 and 5) or are embedded in or shielded with refractory in otherembodiments. Materials suitable for use in the fabrication of theregenerative bed 38 are known in the art.

The second embodiment of the present low NOx regenerative burner isillustrated in FIG. 3. FIG. 3 is a partial illustration of a burner 50having a port block 52 and a port 54. The air uptake and heat storagebed are identical to those of the first embodiment of the invention, andthese structures therefore do not appear in detail in FIG. 3. Combustionis fueled by fuel apertures 56, which inject the fuel into the port 54in the angled fashion as shown. (As an alternative to fuel apertures inthe surrounding refractory, conventional fuel lines and nozzles may beused.) Although two fuel apertures 56 are shown in FIG. 3, additionalfuel apertures may be provided in an evenly-spaced, concentric planararrangement including 4, 6, 8 or 10 fuel apertures, for example. Theforward angled fuel apertures function to induce recirculation ofcombustion gases back to the primary combustion zone, depressing NOx.The fuel jets also entrain combustion air promoting the mixing of fueland air and affecting flame shape and characteristic. The design can bealtered with respect to the angularity and direction of the individualfuel jets to vary the flame shape and characteristic to suit specificrequirements. The number and arrangements of the jets (angularity to thecenterline of the burner and spin angle) can be varied to control thedegree of NOx suppression as well as flame shape and characteristic. Themultiple jet arrangement shown, used in conjunction with individualautomated shut-off valves between the supply manifold and each jet,offers the additional advantage of the ability to reduce the number ofjets as fuel demand reduces in order to maintain entrainment energy andmixing energy. For example, a 6 jet arrangement would permit shut off oftwo jets at 2/3 flow and four jets at 1/3 flow maintaining maximum jetenergy on the remaining active jets. Although opposite fuel apertures 56as shown in FIG. 3 inject the fuel at relative 90° angles, injection maybe effected at relative angles between about 30° and about 150° relatedto the centerline of the burner and also could be provided with spinaction through the use of a second angle to the injection point.

FIGS. 4 and 5 illustrate third and fourth embodiments of the present lowNOx regenerative burners in which combustion is staged. Staged fuelaccomplishes staged combustion in the burner of FIG. 4; staged aireffects staged combustion in the burner of FIG. 5. Referring first toFIG. 4, the burner 60, having a regenerative bed 61 and a port 62,includes first stage fuel apertures 64 and second stage fuel apertures66. The fuel supply to the first stage fuel apertures 64 is limited sothat between 30-70% of the fuel is injected by the first stage fuelapertures 64. Second stage fuel apertures 66, positioned between thefirst stage fuel apertures 64 and the port 62, inject the balance of thefuel (30-70%) into the combustion site. Combustion air for the stagedcombustion enters via the regenerative bed 61. This two-stagearrangement functions to reduce NOx formation not only as a result ofthe combustion gas and combustion air entrainment induced by the pairedapertures 64 and 66, but also as a result of the presence ofconsiderable excess air at the site of the first stage of combustion,which reduces the temperature in the primary combustion zone andsuppresses NOx formation. Although two sets of two fuel apertures eachare shown for the purpose of this third embodiment: of the invention,more than two fuel nozzles at each of the two stages of combustion maybe used, preferably in an evenly-spaced, planar concentricconfiguration. As with the other embodiments of the present invention,the burner 60 is adapted to function, when the flow in the furnace isreversed, as a flue.

Referring now to FIG. 5, the burner which effects staged combustion withstaged combustion air is illustrated in pertinent part. The burner 70having a regenerative bed 72 has a fuel line 80, fuel nozzle 82 (bothembedded in refractory or otherwise insulated or cooled) and a port 78.Combustion air entering the burner 70 via the regenerative bed 72 mixeswith fuel in two stages by means of the primary air passages 74 and thesecondary air passages 76. Primary and secondary combustion areaccomplished by initial provision, through primary air passages 74, ofonly 30-70% of the combustion air at the site of the fuel nozzle 82. Theremaining 30-70% of the combustion air travels via secondary airpassages 76 to effect secondary combustion in the port 78. This stagedair combustion device operates fuel rich in the primary combustion zone,reducing flame temperature and thus suppressing NOx formation. Apreferred construction would be that the air staging structure would beconstructed of suitable ceramic material, due to the elevatedtemperature to which it is exposed. The arrangement of the air apertures76, could be adjusted as to the number of holes, length of holes,direction and spin angle to provide minimum NOx and control of flameshape and characteristic. It is to be noted that despite the relativelymore restricted air flow through primary and secondary air passages 74and 76, as compared with the air flow structures illustrated in FIG. 4,both embodiments of the invention are suitable for use in regenerativesystems and both burners can function as flues when the direction of theflow in the furnace is reversed. Vitiated air provided by the methodshown on FIG. 1 at the fan inlet can be applied to the embodimentsdescribed in FIG. 2, FIG. 3, FIG. 4, and FIG. 5 and this will furthersuppress NOx substantially below the levels possible with theseembodiments alone.

FIGS. 6 through 10 (and also FIG. 1) illustrate various ways to depressNOx formation by using vitiation of combustion air with products ofcombustion. Referring now to FIG. 6, the fifth embodiment of theinvention is illustrated in which a burner 90, having an air intake 94and a regenerative bed 92, is fitted with six converging fuel lines 96,six venturies 98, and six vitiation ducts 100. The burner 90 leads to afurnace chamber via a port 101. By a total of 6 converging fuel lines,therefore, fuel is provided to the burner via its respective venturies98 within the burner refractory. (For the purpose of the presentinvention, a venturi is a hollow area having flared cylindrical shape.)The injection of the high pressure fuel from the fuel lines 96 generatesa negative pressure region in the venturies 98, which negative pressureregion induces furnace gases back from the furnace chamber through thevitiation ducts 100 to the site of combustion. Recirculation of productsof combustion in this manner cools the flame and reduces NOx formationduring combustion. If necessary, during the off cycle when no fuel gasis passing through the fuel lines 96, a small quantity of combustion airand/or recirculated products of combustion can be passed into the burner90 to maintain cooling and to prevent cracking of any stagnant gaseoushydrocarbons present. The burner 90 functions efficiently, in reverse,as a flue.

Although not illustrated in FIG. 6, optionally the vitiation ducts 100,the venturies 98 or the converging fuel lines 96 may be angled forwardor backward or with a tangential component to generate a hyperboloidstream of fuel and induced products of combustion. The angles and spiralcomponents are specifically designed to suit different applications.These variations may be employed to alter the flame shape and geometryto suit a specific application. These options may also be exercised withrespect to FIGS. 2, 3, 7, 9 and 10 herein.

FIG. 8 is a sectional view taken along line VIII--VIII of FIG. 6. Thesix evenly-spaced converging fuel lines 96 and their respectiveventuries 98 can be readily seen in their "planar concentric"configuration.

FIGS. 7, 9 and 10 illustrate additional embodiments whereby products ofcombustion are recirculated for NOx suppression. FIG. 7 illustrates aburner 110 having an air intake 114 in series with the regenerative bed112. The burner 110 incorporates venturies 120, 122 and vitiation ducts124 similar to those of the previous embodiment, but supplements thesestructures with the first and second converging fuel lines 116, 118 asshown. The first converging fuel lines 116 are disposed within firstventuries 120 and the second converging fuel lines 118 are disposedwithin the second venturies 122 to create multiple converging fuelstreams; the pressure phenomena (negative pressure region) generated bythe fuel streams within the respective venturies function to induceproducts of combustion back through the vitiation ducts 124 as effectedby the previous embodiment of the invention Additionally, fuel may bestaged between the first converging fuel lines 16 and the secondconverging fuel lines 118 for further control of flame shape andcharacteristics. Recirculation of products of combustion into the burnercontributes to NOx minimization. More particularly, the collision of thestreams along with the variance in the staging of the fuel--andcommensurate recirculation of products of combustion--creates turbulencelevels which can adjustably determine flame geometry.

FIG. 9 illustrates a sixth embodiment of the invention similar to theembodiment illustrated in FIG. 7. A burner 130 includes an air intake134 in series with a regenerative bed 132. Combustion is effected bymeans of first converging fuel lines 136 and second converging fuellines 138. Each set of immediately adjacent first and second convergingfuel lines 136, 138 converges the fuel at a relative angle greater than30°, such as the relative 45° angle as shown. The collision of theinjected fuel, along with variance in the quantity of fuel exiting eachnozzle, creates turbulence levels which can adjustably determine flamegeometry. The convergence chamber 140 does not itself inducerecirculation of products of combustion; for the purpose of this sixthembodiment of the invention, combustion air via air intake 134 isvitiated with products of combustion recirculated via appropriatefluid-channeling means (not shown), including but not limited to therecirculation means illustrated schematically in FIG. 1 herein or,alternatively as disclosed in U.S. Patent Application Ser. No. 025,365to Finke, entitled "LOW NOX Radiant Tube Burner and Method,"incorporated herein by reference.

Referring now to FIG. 10, combustion air entering the burner 150 firstpasses through the regenerative bed 154 in series with the air intake152 as shown. The linear fuel lines 156 are each paired with coaxialannular fuel lines 158. (Annular fuel lines 158 may have a perforateannular nozzle, not shown, if desired.) Each pair of combined coaxialfuel lines, which yield coaxial streams, lead into the respectiveventuries 160. Although the coaxial fuel lines may obviously create acoaxial fuel/fuel stream, fuel/air and fuel/products of combustionstreams are also contemplated within the scope of the present invention.The coaxial streams create a negative pressure region in the venturi 160which in turn induces recirculation of products of combustion from thefurnace through the vitiation ducts 162. For the purpose of this eighthembodiment of the invention, the fluid exiting the annular fuel line 158is most preferably a low pressure cold air having an energy sourcetherein, which would provide a cooling media to the linear fuel line 156to promote structural stability thereof. (Low pressure cold air energysources may also be used in the other embodiments of the presentinvention where appropriate.) As with the previously describedembodiments of the invention, recirculation of products of combustiondepresses NOx formation.

In all embodiments of the invention disclosed herein, a plurality offuel nozzles in the disclosed positions may be provided, preferably inradial configuration. Moreover, convergence and/or entrainment of thefuel exiting the fuel nozzles may be accomplished with convergenceangles of the fuel nozzles between 30° and about 150°, generally.

Typical fuels for use in these regenerative systems include gas and oil.Refractory materials are well known in the art and are generally ceramiccompositions prepared to specifications required for particular processapplications.

Although the invention has been described in connection with specificmaterials and specific embodiments, the invention is to be limited onlyinsofar as is set forth in the accompanying claims.

I claim:
 1. In a regenerative burner having a regenerative bed, a burnerport and a fuel nozzle, the improvement comprising: a burner bafflehaving apertures therein for selectively directing combustion air andinducing combustion gas recirculation into a primary combustion zone forsuppressing NOx emissions, said baffle and said fuel nozzle beingpositioned substantially adjacent said burner port and beingsubstantially coplanar in a plane perpendicular to a burner axis.
 2. Theregenerative burner according to claim 1 wherein said burner bafflefurther comprises a generally cylindrical baffle having four aperturestherethrough.